The present invention relates to a vehicular headlamp, particularly, to a projection-type headlamp. More particularly, the present invention relates to a headlamp employing a synthetic resin lens in which the lens is protected over an extended service period against discoloration (yellowing) and other problems such as cracking caused by ultraviolet irradiation from a light source.
In order to configure a vehicular headlamp so that it conforms to the overall vehicle design, the outer lens, which greatly affects the vehicle appearance, is often required to have a complicated design. When using an outer lens made of glass, the weight of a large-sized lamp unit, which is a combination of many components, is high. In order to solve the aforementioned problem, an outer lens made of a synthetic resin material has been increasingly employed as it can be more readily formed into a required shape and is lighter in weight than an outer lens made of glass.
However, a synthetic resin lens is susceptible to damage by ultraviolet radiation. That is, when such a lens is exposed to ultraviolet radiation for a long period of time, discoloration or cracking is likely to occur.
Particularly, a discharge bulb emits large amounts of ultraviolet radiation. As a result, in a headlamp employing such a bulb as a light source, it is important to take measures to protect the synthetic resin lens from exposure to such ultraviolet radiation.
In one type of conventional vehicular headlamp using a discharge lamp bulb as a light source, for example, a projection-type headlamp, an ultraviolet absorbing film is formed on the inner surface of the projection lens of the projection unit. The ultraviolet absorbing film absorbs ultraviolet radiation contained in the light rays emitted by the discharge lamp bulb so as to protect the synthetic resin lens from exposure to ultraviolet radiation.
FIG. 3 shows a conventional projection lens, the inner surface of which is coated with a ultraviolet absorbing film. More specifically, as illustrated in FIG. 3, an ultraviolet absorbing film c is formed on the inner surface b of a projection lens a. The ultraviolet absorbing film c is formed by applying a coating film (paint), which is prepared by kneading zinc oxide e with a resin d as a base material, on the inner surface b. In FIG. 3, the thickness of the ultraviolet absorbing film c is shown exaggerated relative to the other components.
The conventional projection lens a having an ultraviolet absorbing film c coated on the inner surface thereof, however, suffers from various problems. For example, the ultraviolet absorbing film c will deteriorate after use for a long period of time owing to the low heat resistance of the base resin d constituting the coating film, thus losing its ultraviolet absorbing effect. The projection lens a has rather low transparency as the ultraviolet absorbing film c contains resin, thus deteriorating the light distributing control properties of the lens. Furthermore, as the viscosity of the base resin d has to be made high so as to cause the paint to adequately adhere to the lens, spots tend to be formed in the ultraviolet absorbing film c due to an uneven thickness.